1. Field of the Invention
The present invention relates, in general, to optical disc drivers and, more particularly, to an optical pickup having a correction unit for correcting an aberration resulting from a tilt of an optical axis of a beam of light, which is emitted by the optical pickup and focused onto the recording surface of an optical disc, relative to the recording surface.
2. Description of the Prior Art
It is commonly required that an optical pickup should be improved in precision to meet requirements for a densification of an optical disc. In particular, it is currently required that an optical axis of a beam of light, which is emitted by the optical pickup and focused onto the recording surface of an optical disc, that is, an optical axis of an object lens installed in the optical pickup should be perfectly perpendicular to the recording surface.
On the other hand, with the densification of the optical disc, the number of apertures (NA) of the object lens installed in the optical pickup increases and the beam of light emitted from the optical pickup has a shorter wavelength. As a result, a coma-aberration increases significantly due to a tilt, which leads to a keen need for correction of the coma-aberration.
In the prior art, a liquid crystal panel has been used as a correction unit for correcting such a coma-aberration. A conventional liquid crystal panel has an electrode layer on each surface of its liquid crystal substrate to change the orientation of liquid crystal molecules in response to voltages applied to the electrode layers. The liquid crystal panel thus changes the refractive index of a bean of light passing through its liquid crystal substrate, and corrects a wavefront aberration, caused by a tilt of an optical axis of a light beam emitted by an optical pickup. That is, the voltages, applied to the divided areas of the liquid crystal substrate, are changed to cause a change in the refractive index of the light beam. It is thus possible to make the lengths of the optical paths at the divided areas different from each other, and change the optical axis extending to the recording surface of an optical disc, and thereby correct an aberration due to a tilt of the optical axis of the light beam.
However, the liquid crystal panel used as an aberration correcting unit in the prior art is Problematic as follows. That is, during an operation of a conventional optical pickup using a liquid crystal panel, the object lens used for focusing the light beam to the recording surface of an optical disc is shifted in a radial direction of the disc due to a tracking servo of the optical pickup for radially moving on the recording surface to read data of the tracks of the disc. The optical axis of the object lens may be thus misaligned from the central axis of the liquid crystal panel in the radial direction of the optical disc, and deteriorates the aberration correcting performance of the optical pickup. When both the object lens and the liquid crystal panel are shifted, it is possible to prevent such a deterioration of the aberration correcting performance. However, such a shift of both the object lens and the liquid crystal panel undesirably causes the shifted parts of the lens and the panel to be overlapped. In addition, it is very difficult to precisely arrange the electrodes on the liquid crystal panel in the case of the shift of both the object lens and the liquid crystal panel, and so such a shift has been less likely to be used in the prior art.
In an effort to overcome such deterioration in the aberration correcting performance of an optical pickup, a checkered electrode pattern may be formed on a liquid crystal panel, with the voltage applying points being changed in response to a radial shift of the object lens. However, this technique is problematic in that the liquid crystal panel is excessively expensive, in addition to causing an operational problem while controlling the liquid crystal panel.